The present invention relates to a method of treatment for improving the quality of a ferroelectric liquid crystal device. More particularly, the invention relates to a method of treatment for preventing image sticking that occurs when a liquid crystal is held in one of its stable states for a long period of time, and for recovering the liquid crystal from image sticking if image sticking occurs at all.
Liquid crystal devices are used not only for display devices but also for spatial light modulators such as optical shutters.
Ferroelectric liquid crystals have two stable states, hereinafter called the one stable state and the other stable state. In a liquid crystal device using a ferroelectric liquid crystal, an image is displayed, or the device is operated as an optical shutter, by switching each pixel from the one stable state to the other stable state.
Ferroelectric liquid crystals have the property that when an applied voltage is removed, the immediately previous state, i.e., the one or the other of the stable states, is retained (this property is hereinafter called the xe2x80x9cmemory propertyxe2x80x9d). Accordingly, the ferroelectric liquid crystal, once set in a stable state, remains in a stable state until a voltage greater than the threshold is applied for a certain length of time.
Further, since ferroelectric liquid crystal molecules each possess spontaneous polarization, when the ferroelectric liquid crystal molecules in a pixel are in one stable state, an electric field pointing in one direction occurs in the liquid crystal layer of the pixel.
When one of the stable states is maintained for a long time, for example, a few milliseconds or a few seconds, ionic impurities in the liquid crystal layer agglomerate by being attracted in the direction of the electric field. This disrupts the electric symmetry in the liquid crystal layer. As a result, when the liquid crystal is switched from one stable state to the other stable state, a portion of the liquid crystal returns to that one stable state and contrast degrades. This phenomenon is called the xe2x80x9cimage sticking phenomenonxe2x80x9d. This phenomenon is a factor that works to degrade the quality of the liquid crystal device. For example, when the ferroelectric liquid crystal device is used as a display device, if the image sticking phenomenon occurs, the previously displayed image remains visible, degrading the display quality.
To prevent such image sticking, it is imperative to avoid setting all the ferroelectric liquid crystal in the pixel in the same state when leaving the ferroelectric liquid crystal in the non-driven state, i.e., in one of the stable states, for a long period of time. That is, the electric field formed in the liquid crystal layer by the spontaneous polarization must be canceled by having the two stable states exist simultaneously.
Japanese Patent Unexamined Publication No. 2-225592
discloses a technique which involves adding an impurity ion adsorbing material to the liquid crystal in order to prevent the occurrence of image sticking. However, it is not always possible to adsorb all the impurity ions.
Further, Japanese Patent Unexamined Publication No. 2-165122 discloses a technique which periodically applies positive and negative electric fields during the non-driven period to switch the direction of the liquid crystal alignment from one direction to another. This method, however, not only requires a complex circuit configuration, but does not function property if, for some reason, the power is cut off.
On the other hand, once image sticking has occurred, the liquid crystal does not recover by itself if the liquid crystal is driven in the usual way; therefore, a treatment for recovery becomes necessary. One known method of recovery treatment involves the heat treatment hereinafter described. The temperature of the liquid crystal is first raised to the temperature of the isotropic phase, and then the temperature is gradually lowered to the temperature of the nematic phase; this causes the long axes of the molecules to align in the same direction. When the temperature is further lowered to the temperature of the smectic A phase, not only the molecular long axes but also the molecular centers of mass align. When the temperature is further lowered into the smectic C phase, the molecules are realigned in the original state. As a result, the image sticking is removed, and the quality of the liquid crystal device is thus recovered. This series of heat treating operations is called the isotropic treatment.
A ferroelectric liquid crystal device designed to perform the isotropic treatment is equipped with a plate-like heating element for heating and a temperature sensing element for sensing the temperature of the liquid crystal so that the temperature can be raised to the temperature of the isotropic phase.
The prior art isotropic treatment, however, requires heating the whole panel to heat all the liquid crystal elements to the temperature of the isotropic phase and then gradually cooling it to the operating temperature. This clearly reduces time efficiency and increases power consumption.
An object of the present invention is to provide a simple and reliable method of treatment for image sticking prevention to prevent the image sticking that occurs when the ferroelectric liquid crystal in a ferroelectric liquid crystal device is held in one of its stable states for a long time. Earlier, the length of time to hold the liquid crystal in one of the stable states was described as being a few milliseconds or a few seconds. In contrast, in the present invention, image sticking can be prevented even when the liquid crystal has been held in one of the stable states for a few hours or tens of hours.
Another object of the present invention is to provide a simple and reliable method of recovery treatment for recovering the ferroelectric liquid crystal device from image sticking if image sticking occurs at all in the ferroelectric liquid crystal device.
To achieve the above objects, according to the treatment of the present invention, a low-frequency AC voltage is applied to the ferroelectric liquid crystal device, thereby forming multiple domains in a pixel, causing domain boundaries to flow, and inducing fine segmentation of domain regions.
(Operation)
In operation, the treatment for preventing the occurrence of image sticking is performed in the following manner.
In the ferroelectric liquid crystal device, when leaving the liquid crystal in one of its stable states for a long time, an AC voltage, whose frequency is distinctly lower than that of the usual liquid crystal driving waveform and whose amplitude does not exceed the threshold of the liquid crystal, is applied in advance to the liquid crystal device. For example, an AC voltage having a frequency of 10 to 100 Hz and an amplitude of xc2x11.5 to xc2x13.0 V is applied for about one minute. As a result, domains consisting of two stable states are formed within a pixel, and the domains are repeatedly segmented with the domain boundaries flowing in an irregular manner, eventually resulting in the formation of finely dispersed multiple domains.
In the ferroelectric liquid crystal device treated as described above, when the liquid crystal was driven in the usual way after leaving it in one of its stable states for a long time, image sticking did not occur.
On the other hand, the treatment for recovering the liquid crystal from image sticking is performed in the following manner.
In the ferroelectric liquid crystal where image sticking has occurred after being left in one of its stable states for a long time, an AC voltage, whose frequency is distinctly lower than that of the usual liquid crystal driving waveform and whose amplitude does not exceed the threshold of the liquid crystal, is applied in the same manner as earlier described. For example, an AC voltage having a frequency of 10 to 100 Hz and an amplitude of xc2x11.5 to xc2x13.0 V is applied. As a result, domains consisting of two stable states are formed within a pixel. Then, the domains are repeatedly segmented with the domain boundaries flowing in an irregular manner, eventually resulting in the formation of finely dispersed multiple domains.
In the ferroelectric liquid crystal device treated as described above, image sticking did not occur in the subsequent liquid crystal driving, and it was observed that the electric symmetry in the liquid crystal cell had been nearly perfectly restored.
Using the treatment method of the present invention, the image sticking phenomenon that would occur in the ferroelectric liquid crystal when the liquid crystal was held in one of its stable states can be prevented in a simple and reliable manner, by applying the low-frequency AC voltage.
Further, by using the treatment method of the present invention, recovery from image sticking can be accomplished in a short period of time in a simple manner, by applying the low-frequency AC voltage and without specifically requiring the provision of heating means or the like. Accordingly, by incorporating the low-frequency AC voltage application means into the liquid crystal device together with image sticking detection means, image sticking of the liquid crystal device can be prevented and, further, recovery from image sticking can be accomplished. As a result, the liquid crystal device of high quality can always be obtained without depending on the history of the liquid crystal state.